Wireless networking

Dear Friends ... when in hotel or airport we access the wifi ... no password set to access wifi (WPA, WEP, WPA2).... but when we open our browser for using internet .... a page redirect us and ask for user name and password ... can any one give me just idea how can we do that ...

This is a process known as a 'Captive Portal' and is common on public WiFis.

You have a number of options to deploy such a device, some involve the use of DNS poisoning until you have been validated whilst others use some custom firmware and software for the router. The router option is most robust, and open source packages such as:

DD-WRT
Tomato
OpenWRT

Should allow you to do such a thing, however it can be a daunting and potentially risky task, risky in that if it fails you might brick (break it beyond repair) your router

There are a number of tutorials that can be found with a quick web search however you firstly need to find out what hardware you have (the router) as to find what firmware you can run.

The different firmware (of which I only listed three out of the hundreds) you can install shall depend on the type of router you own. Only some routers shall work because of the way they are built and the hardware that they contain inside. Each website should list all the available routers that it shall run on, if your router is listed then you are in luck.

Once you've selected your firmware, the website should contain the necessary documentation and most likely a forum to get it setup and running. Once you have that sorted, it should just be a case of finding the Captive Portal option on the new control panel that is installed, filling in a couple of fields and clicking save.